As a conventional art, JP-2002-357465-A, for example, discloses an airflow meter.
The airflow meter is for measuring an intake air amount of an internal combustion engine for example. As shown in FIG. 8, the airflow meter has a bypass passage 100 to introduce a part of the intake air and a sensing portion 110 disposed in the bypass passage 100 to measure an airflow amount.
The bypass passage 100 has a restriction portion 120 having a three-dimensional structure to gradually decrease a cross-sectional area of the passage in an airflow direction. By providing the restriction portion 120, a turbulence of the air introduced in the bypass passage 100 is limited, so that it becomes possible to obtain a stable output.
However, by providing restriction portion 120 in the bypass passage 100, the pressure loss large influences much in a small airflow condition when the intake air amount is small. Further, the cross-sectional area increases from a narrowest portion having the smallest cross-sectional area in the restriction portion 120 at a downstream side, generating a separation in the airflow (an eddy) which flows from a center portion toward an inner wall in the passage. Thus, in the small airflow condition, an airflow speed is decreased and fluctuated to cause an issue of spoiling a measuring accuracy.
Against this issue, as shown in FIG. 9, it is possible to decrease an influence of the pressure loss by shaping of the restriction portion 120 in such a manner of cutting off a downstream side thereof, namely terminating the restriction portion 120 at the narrowest portion.
However, by shaping the restriction portion 120 as shown in FIG. 9, the cross-sectional area intensively increases at the terminal end of the restriction portion 120, to generate the separation in the airflow (an eddy) and the turbulence of the air more than a structure shown in FIG. 8. Thus, it may rather spoil the measuring accuracy by the sensing portion 110 disposed in the restriction portion 120. Further, in a case that the air introduced in the bypass passage 100 flows upward as shown the figure, a downstream end face of the restriction portion 120 (a flat portion 130) is disposed upward, causing an issue that dusts sediment on the flat portion 130.